As is known, picture signals in general have a luminance or brightness signal component, which determines the brightness level of pixels at any particular time, and a chrominance or colour signal component, which determines the colour level of pixels at any particular time. In for example the television formats NTSC, PAL and SECAM, a composite colour video signal includes both luminance and chrominance signals. The way these signals are generated and used differ in detail between the different television formats, but those differences are not material for present purposes. Moreover, the terminology used for the different signals differs between the different formats. Unless the context requires otherwise, the term “luminance” and the term “chrominance” used herein apply equally to the “brightness” and “colour” signals of the composite video signals of any of these television formats and any other equivalent format that uses luminance and chrominance signals.
As is known, in order to achieve backward compatibility with monochrome (black and white) television sets at the time, the colour television formats were set so that the chrominance signal has a narrow bandwidth compared to the luminance signal. The effect of this in practice is that chrominance transitions around colour edges are slower than luminance transitions around the corresponding luminance edges. The way that this manifests itself in the displayed image is that, at an edge in an image, whereas the luminance might for example shift rapidly from a bright region to a neighbouring dark region, the corresponding shift in the chrominance is slower such that there is often a mismatch between the chrominance edges and the luminance edges. This becomes displayed as hue distortions around colour edges such that, for example, red or blue colour can “bleed” into an adjacent region that should be pure white, causing a distortion in the colour of the “white” region at the colour edge. In early colour televisions, this difference was barely visible owing to the relatively poor quality of colour reproduction at the time. However, with more modern television sets, including particularly the large sized displays that can be achieved with LCD and plasma display panels, the visibility of hue distortions around the colour edges has become a more significant problem.
A number of colour enhancement methods are known. For example, in U.S. Pat. No. 5,920,357, there is disclosed a colour transient improvement method in which an enhanced chrominance signal is obtained by selecting one of three signals at different times. The three signals are the original chrominance signal, a derivative of a once-delayed original chrominance signal, and a twice-delayed original chrominance signal. A choice is made between these three signals according to their relative values in order to avoid overshoot and undershoot. However, this method is rather basic and does not produce optimal results.
Another example of colour transient improvement is disclosed in US-A-2003/0107678. The method of this prior art is shown schematically in FIG. 1. An input chrominance signal 1 is passed through a Gaussian filter 2 to remove noise. Then a second derivative with respect to time is taken 3. Using the sign of the second derivative, a correction signal 4 is created. In particular, the sign of the second derivative determines if the correction signal 4 is derived from a local minimum or a local maximum of the chrominance signal. The absolute value of the second derivative is used in a gain control generator 5 to generate a gain control function accordingly. Then, using the correction signal and the gain control function, a chrominance adjustment signal is added to the filtered original chrominance signal in an adder 6 to generate the final output chrominance signal 7. A particular problem with the method of US-A-2003/0107678 is its use of and reliance on the second derivative of the original chrominance signal. This means that in the case that transitions in the chrominance signal are linear, no changes to the chrominance signal will be effected by this prior art method. Moreover, the enhanced colour transitions are still sometimes not aligned with the luminance edges and, in fact, the hue change artefacts around the edges can be made even more visible using this prior art method.